Traditional switching power supplies use feedback compared to an internal reference in order to control the output to within a specified range. From a DC standpoint this is ideal since the output is actually controlled to the desired accuracy. From an AC/transient standpoint this is less than ideal because of the finite time required to respond to an “input” change.
In this case “input” refers to any one of the factors that can affect output voltage of the power supply, including:
Input voltage;
Power switch drive voltage;
Output current;
Power supply temperature;
Power supply frequency.
Input voltage and output current can change very rapidly. Traditional output voltage feedback has no way of detecting that change until it has already resulted in a change in output voltage. Additionally, due to hardware limitations and stability criteria the control system cannot respond instantly to a change in output voltage.
The net result is a compromise between fast response and unconditional stability. Often this is adequate, but in some cases a feed forward element must be incorporated to achieve desired performance. An example of this is in current mode control which has inherent input voltage feed forward. This is especially helpful in applications that may rapidly switch from one input source to another, such as a notebook computer.
Another situation in which feedback is less than optimum is in high output load transient situations such as those encountered in high performance microprocessor power supplies. Until now there has been no satisfactory feed forward methodology developed to deal with this problem so practical solutions focus on minimizing the time it takes the power supply to respond to a change in output voltage.